1. Field of the Invention
The invention relates to a termination circuit, and more particularly to a termination circuit of a plurality of memories.
2. Description of the Related Art
As system bandwidths continue to increase, memory technologies have been optimized for higher speeds and performance. The next generation family of double data rate (DDR) synchronous dynamic random access memory (SDRAM) is the DDR3 SDRAM. DDR3 SDRAMs offer numerous advantages compared with DDR2 SDRAMs, such as lower operation power, higher speeds, higher performance (twice the bandwidth) and so on. Specifically, DDR3 SDRAM has reduced power consumption when compared with DDR2 SDRAM, primarily due to smaller die sizes and a lower supply voltage (e.g. 1.5V for DDR3 SDRAM versus 1.8V for DDR2 SDRAM).
Unlike DDR2 SDRAM, which uses a T-branch topology, DDR3 SDRAM adopts a fly-by topology that provides better signal integrity at higher speeds. The fly-by topology is applied in the command, address, control and clock signals for the DDR3 SDRAM. The signals from the memory controller are connected in series to each DRAM device, thereby improving signal integrity by reducing the number of stubs and the stub lengths.
A termination circuit typically provides a termination resistance to a common node of a plurality of DDR3 SDRAMs, so as to prevent degradation of signal integrity on transmission lines for the DDR3 SDRAMs. The common node may be at ground potential or at a potential half way between a ground and a supply voltage, wherein the required termination voltage can be regulated. For example, a memory termination circuit of a DDR DRAM can be required to provide a termination voltage approximately equal to one-half of a supply voltage, such as VDD, at the common node. However, conventional termination circuits, such as conventional DDR DRAM termination circuits, suffer from various disadvantages.
In a conventional termination circuit, each transmission line is driven by a driver and is coupled to a regulated voltage node. The regulated voltage node is coupled to a ground through a termination resistor and a termination voltage regulator connected in series, wherein the termination voltage regulator is a special regulator that can source and sink current. Although the termination voltage regulator can provide a precise termination voltage at the regulated voltage node, the termination voltage regulator is a separate component that can undesirably increase manufacturing costs.
Therefore, a termination circuit without any termination voltage regulator is desired.